Thunderstorm Rainfall in the Conterminous United States

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1 Thunderstorm Rainfall in the Conterminous United States Stanley A. Changnon Changnon Climatologist, Mahomet, Illinois ABSTRACT Thunderstorm rainfall amounts during the period were determined for 51 first-order stations distributed across the United States, and these values were assessed for seasonal and annual variations in both space and time. Thunderstorms produce 48% of the average annual precipitation received in the Mississippi River basin, which embraces 41% of the United States. Hence, the temporal and spatial variations in thunderstorm rainfall are major factors affecting most of the nation s water cycle. However, thunderstorm rainfall is only a small part of the total precipitation on the West Coast, typically less than 10% of the annual total. Thunderstorms maximize in the summer in most areas and produce 72% of the total summer rainfall that occurs east of the Rocky Mountains. Thunderstorm rainfall in the spring exceeds summer values in the southern plains and portions of California with 40% of the annual average storm precipitation, but spring values rank second in most areas of the nation. Thunderstorm rainfall amounts explain at least 50% of the variation found in annual total precipitation across large portions of the nation. Thunderstorm rainfall departures below average in the driest five years were found to closely match those of total precipitation deficiencies at all stations, revealing that the absence of storm rainfall is a major factor in droughts. Similarly, the magnitude of thunderstorm rainfall departures in the five wettest years of matched the magnitude of the total precipitation departures, revealing that thunderstorms have a sizable influence in producing extremely wet as well as dry years across the nation. The temporal distribution of thunderstorm rainfall during showed 10% 55% increases over time in most parts of the nation except the upper Midwest and a small portion of the Southeast. Increases were statistically significant in the northern high plains and intermountain area of the West. Trends of storm days with heavy rainfall were also upward across the entire nation, being sizable on the West Coast, Intermountain West area, and Northeast. The national pattern based on temporal shifts in thunderstorm rainfall is in agreement with that based on shifts in storm frequencies indicating that the temporal increases in storm rainfall were a result of more thunderstorms over time and more storm days with heavy rainfall. 1. Introduction Thunderstorms are a fundamental component of the nation s climate, serving as a key element in the water cycle and the global atmospheric electric circuit. As a result, numerous in-depth studies have assessed the physical nature of thunderstorms, their prediction, and their climatology. Thunderstorms and the phenomena they produce lightning, tornadoes, hail, high Corresponding author address: Stanley A. Changnon, Changnon Climatologist, Mahomet, IL In final form 3 April American Meteorological Society winds, and heavy rainfall have received great attention because of the significant damages they produce. For example, thunderstorms during the period caused $87 billion in damages (1998 dollars) to property in the United States (Changnon 2001a). Thunderstorm damages rank just behind floods and hurricanes as the nation s most damaging weather condition (Changnon 2001b). Figure 1 reveals that thunderstorms occur in all parts of the nation, but the average annual number of thunderstorms varies widely, being greater than 70 days with storms at weather stations in Florida and being less than five storms a year at points along the West Coast. Surprisingly, very little attention has been given to the amount of precipitation produced by thunderstorms and to measuring the environmental impacts Bulletin of the American Meteorological Society 1925

2 FIG. 1. The pattern of the average annual number of thunderstorm days based on data for at the 51 stations assessed in this study. of this precipitation. The benefits are likely very large but not well defined. For example, thunderstorm rainfall contributes all of the annual runoff measured on small river basins in Arizona and New Mexico (Osborn and Reynolds 1963), and represents 75% 80% of the average growing season rainfall in Illinois (Changnon 1957). Ironically, the first major meteorological and climatological assessment of thunderstorms, which was entitled Thunderstorm Rainfall, did not present any estimates or data on the actual amounts of rain that storms produce (Hydrometeorological Section 1947). Another major climatological study of thunderstorms also did not present values of thunderstorm rainfall (Court and Griffiths 1981). These two studies compared average thunderstorm frequencies with the average amount of total precipitation on a monthly and/ or annual basis, inferring that a close relationship existed. An approach used in other thunderstorm rainfall studies compared thunderstorm frequencies with total precipitation as defined at different daily levels, such as 0.25 mm day 1 or h 1 (Wallace 1975). All such prior approaches assumed that thunderstorms produced a significant amount of the total precipitation without specifying how much. However, a few studies have reported on actual amounts of rainfall derived from thunderstorms. One was based on a study of Illinois data (Changnon 1957), and a second on values of thunderstorm rainfall for an Arizona river basin (Osborn and Hickcock 1968). Thunderstorm rainfall amounts determined from 30 yr of data at first-order stations across the nation were employed to statistically define regions with similar rainfall distributions (Easterling 1989), and to assess the probabilities for occurrence of three levels of thunderstorm precipitation in each season (Easterling and Robinson 1988). These prior studies of thunderstorm rainfall did not present information on the seasonal and annual averages and extreme amounts of thunderstorm precipitation including the rainfall patterns across the United States, nor did they assess temporal variations in thunderstorm rainfall. The objective of this study was to determine the rainfall produced by thunderstorms across the conterminous United States, to define the relationship between thunderstorm rainfall and total precipitation, and to examine the temporal distribution of thunderstorm rainfall. Hourly and daily values of rainfall at times when thunderstorms occurred were determined for a 45-yr period, These data were used to derive seasonal and annual measures of thunderstorm rainfall. 2. Data Daily and hourly precipitation data for the period from 51 first-order stations distributed across the nation were analyzed to determine the thunderstorm rainfall. For every hour and day when thunderstorms were reported, the associated hourly and daily amounts were recorded, and monthly, seasonal, and annual totals were derived for the stations shown in Fig. 1. Any historical analysis of thunderstorms must address questions about the quality of the data. Thunderstorms are recorded based on hearing thunder and this has the potential for inconsistent record keeping. Audibility of sound is affected by atmospheric conditions, noises, and barriers between the source of sound and the listener. The historical thunderstorm data collected at all first-order stations with 90-yr or longer records had been evaluated using a five-step procedure, showing that several stations had questionable data (Changnon 2001c). Hence, this study was based on data at 51 stations that had been found to have quality records. The period of record selected for analysis, , was based on available storm data including hourly and daily values of thunderstorms and precipitation. Thunder data in the latter part of the 1990s are considered suspect. The 51 stations selected do not define all the features in the nation s thunderstorm occurrences, particularly in some western mountain areas where high values are underestimated. However, the 51 stations are distributed across the nation and sample most of the climatic differences Vol. 82, No. 9, September 2001

3 Daily precipitation amounts on thunderstorm days may include some precipitation not derived from the thunderstorms, such as rain from adjacent showers or general rains occurring before or after a storm and on the same day. Hence, the daily precipitation amounts on thunderstorm days represent an overestimate of the true storm rainfall. Hourly rainfall values for the hours just before and after observation times when thunderstorms were reported more closely approximate the rain that is storm produced. Hourly thunderstorm reports are based on conditions existing at the end of each hour when observations are taken. Hence, in this analysis the hourly amounts assigned to thunderstorms included those that occurred in the hour just before a thunderstorm was reported and those in the hour after each thunderstorm report. Hourly amounts so determined would appear to be a better estimate of the thunderstorm rainfall than the daily amounts on a thunderstorm day. However, investigation of the hourly records of thunder occurrences at the 51 stations revealed that between 15% and 40% of all days with reported thunderstorms had no reports of thunderstorm in their hourly observations. Thunder occurred but in the time periods between the hourly observations. Table 1 shows the percent of missed thunderstorm occurrences for stations selected from different parts of the nation. These cases occur because the thunderstorms existed but not at the time the hourly observations were taken. This finding meant that the estimation of thunderstorm rainfall using the hourly data would miss a large amount of the thunderstorm rainfall in a given month or year, and would represent a sizable underestimate of the actual thunderstorm rainfall. The use of the daily precipitation totals on days with thunderstorms was assessed. The question became, how sizable was the overestimation of thunderstorm rainfall by using the daily amounts on days with thunderstorms? This was assessed by comparing the 24-h amounts derived from the hourly thunder amounts with the daily totals on just those days when thunderstorms were reported in the hourly record. The seasonal and annual precipitation values based on the hours with thunderstorms were ex- pressed as a percent of the total 24-h values for the 45- yr period of record, and the resulting annual values determined for various stations are shown in column 2 of Table 1. The daily totals determined from hourly values for the 51 stations ranged from 86% to 95% of the amounts computed from the daily totals. These underestimates are much less than the underestimates inherent in the use of just the hourly values as a measure of thunderstorm rainfall. An analytical approach was devised for modifying the daily precipitation totals on days with thunderstorms. The calculation of the average seasonal and annual amounts of thunderstorm rainfall was done using 1) the daily totals on thunderstorm days, and 2) multiplying these values by the long-term percentages determined for each station, as shown in Table 1. For example, the average annual thunderstorm rainfall based on daily totals at Kansas City was 66.6 cm, and this value was modified by 92% (see Table 1) resulting in a revised value of 61.3 cm. Most seasons at the 51 stations were found to have somewhat different values for adjusting the total daily thunderstorm rainfall amounts. For example, the summer season adjustment value determined from the Kansas City data TABLE 1. The annual percent of thunderstorm days with no hourly reports of thunder, and the annual amount of thunderstorm rainfall, as determined from hourly records (on days when thunder was reported in the hourly observations), expressed as a percent of the total daily rainfall on thunderstorm days. Values are based on data. Percent of thunder days Percent of daily storm rainfall Station missed in hourly records in hourly reports Harrisburg Jacksonville Bismarck Denver Kansas City Phoenix Memphis Madison Seattle Bulletin of the American Meteorological Society 1927

4 was 96%, that for winter was 89%, and that for spring and fall was 90%. The seasonal ratios were used to adjust the daily values for each season, and the resulting values were the thunderstorm rainfall amounts for each station. The adequacy of this adjustment approach was further assessed for temporal shifts in the ratios. The seasonal and annual ratios at all stations were computed for the three 15-yr periods in the sample. These revealed minor fluctuations over time, with most shifting less than 5% between periods. For example, the Peoria annual ratios were 0.92 for , 0.88 for , and 0.91 for , yielding a 45-yr mean of The sequence of 15-yr annual ratios at Denver was 0.88, 0.85, and 0.89, respectively. The values revealed sufficient temporal stability to indicate their use would not lead to major distortions of the temporal fluctuations and trends in thunderstorm rainfall over the 45-yr period. The resulting thunderstorm-related rainfall amounts were compared spatially, and with the total seasonal and annual amounts of precipitation. Days when thunder was reported and when no measurable rain occurred were also assessed. Daily average rainfall amounts from thunderstorms were also calculated FIG. 2. Measures of average annual thunderstorm rainfall. for the various seasons and year. Annual amounts of thunderstorm rainfall and incidence of thunderstorm days with heavy rainfall were assessed for their temporal behavior during the period. 3. Spatial results a. Annual values The average annual rainfall produced by thunderstorms (Fig. 2a) is highest, greater than 70 cm, in the Southeast where storm incidences are also greatest (Fig. 1). Miami with 91 cm has the highest average. Values diminish away from this area northward to cm along the Canadian border. Average amounts decrease rapidly westward across the Great Plains, becoming 15 cm along the front range of the Rockies. Thunderstorm-produced annual amounts are least, less than 5 cm, along the West Coast where five or fewer thunderstorms occur in the average each year. The minimum is at San Diego with only 2 cm. Average amounts also decrease across the Northeast, decreasing from 30 cm in western Pennsylvania to 15 cm in Maine. The pattern based on the daily average thunderstorm rainfall for the year (Fig. 2b) does not closely match that for total amounts (Fig. 2a). For example, the highest daily values, those greater than 13 mm, occur in east Texas and the lower Mississippi River valley, an area west of the total rain maximum in Florida. This area of high daily rainfall values is also the area of greatest average frequency of cloud-toground flashes (Changnon 1989), and a positive relationship exists between flash frequency and rainfall in this area (Sheridan et al. 1997). The thunder-day rainfall values on the West Coast exceed 5 mm and most are double those in the Intermountain West region where the daily average storm rainfall amounts range between 2 and 4 mm. Easterling and Robinson (1988) found a high probability for no daily precipitation to occur with thunderstorms in this western mountain area. The pattern based on the average annual thunderstorm precipitation expressed as a percent of the total average annual precipitation is shown in Fig. 3. Stations in the central and southern Great Plains receive between 60% and 70% of their annual total precipitation from thunderstorms. Interestingly, this high is not in the same general area where the daily amounts or total storm amounts peaked, reflecting the influence of having lower nonthunderstorm precipitation in the 1928 Vol. 82, No. 9, September 2001

5 FIG. 3. Average annual thunderstorm rainfall expressed as a percent of the total annual precipitation, plains. The 60% area occurs where elevated thunderstorms are most frequent (Colman 1990). This type of thunderstorm develops above frontal surfaces and these storms are isolated from effects of the surfacebased boundary layer. Thunderstorm contributions to the annual total precipitation diminish slowly from the 60% area to the north and east, becoming less than 20% in the Northeast. Percentages decrease rapidly westward such that values in the mountainous West range between 20% and 45%. Thunderstorms caused by the southwest summer monsoon (Hales 1974) raise the percentages in the Southwest to 40% 50%. Values are below 10% along the West Coast. The average of the 36 stations in the eastern twothirds of the nation is 47%, revealing that thunderstorm rainfall is a significant contributor of the total precipitation over a sizable part of the nation. Figure 4 illustrates the relationship between the annual thunderstorm rainfall and total precipitation at two stations, Kansas City and Helena, two sites with different climates and with major differences in storm frequencies. Helena averaged 33 thunderstorm days per year for , whereas Kansas City averaged 52 days. Thunderstorms produced 66% of the total annual precipitation at Kansas City, compared to 25% at Helena, revealing that the annual total at Kansas City is much more dependent on thunderstorm rainfall. A strong relationship exists for the Kansas City values with a correlation coefficient of 0.93, indicating that the annual amount of thunderstorm rainfall explained 86% of the variability found in the station s total precipitation. However, at Helena, the correlation coefficient was only 0.63, revealing that thunderstorm rainfall was not a major controlling factor in the amount of annual precipitation received there. For example, note in Fig. 4b that when the annual thunderstorm rainfall at Helena varied between 5 and 10 cm, the annual totals varied widely from a low of 18 cm to a high of 42 cm, more than a 2-to-1 difference. The relationship of annual values of thunderstorm day frequencies, thunderstorm rainfall, and total precipitation varied across the nation. Comparison of the values for all stations across the nation revealed the thunderstorm rainfall amounts had a better relationship with the total precipitation than did the number of thunderstorms. The values for a few stations distributed across the nation are shown in Table 2. As ex- FIG. 4. The relationship between annual total precipitation and annual thunderstorm rainfall at Kansas City and Helena. Bulletin of the American Meteorological Society 1929

6 TABLE 2. Correlation coefficients determined for two annual measures of thunderstorm activity and the annual total precipitation. Thunder rainfall and total No. of thunderstorms and total Station precipitation and thunder rainfall precipitation Kansas City Memphis Miami Denver Dallas Bismarck Albuquerque Albany Spokane pected, the number of thunderstorm days related better to thunderstorm rainfall than to total annual precipitation. The correlation coefficients of thunderstorm rainfall with the annual precipitation total were found to be highest, greater than 0.85, at stations in the nation s southern and central regions, and least, less than 0.6, in the western third of the nation. Patterns based on the maximum and minimum annual percentages of the total precipitation from thunderstorms, as experienced during , appear in Fig. 5a. These extremes provide a useful measure of the variability of this measure of the contribution of thunderstorm rainfall. The peak of the maximum values peak is 90%, and these values occur where the average percentages (Fig. 3) were highest. These highest annual values reveal that thunderstorm precipitation had reached 50% or more of the annual totals in at least one year in most parts of the nation except for the West and Northeast sections. The lowest annual percentages found during (Fig. 5b) also reflect the average pattern with values above 50% in the central-southern plains and the lower Mississippi basin. Annual thunderstorm precipitation amounts during had never fallen below 50% of the annual total in this area, nor below 30% over a large section of the south-central United States. This pattern helps reveal where thunderstorm rainfall is a large share of the annual total precipitation, even in years with No. of thunderstorms extremely low thunderstorm rainfall. The pattern based on the average annual number of days with no measurable rain when thunderstorms were reported is depicted in Fig. 6a. A maximum of more than 11 days per year exists in the western high plains and Rocky Mountains, a factor behind the lightning-induced forest fires in this area (Whiteman 2000). The high occurrence of dry thunderstorm days in the west has been attributed to two factors. Court and Griffiths (1981) showed that weather observers at stations at higher elevations, where audibility of sound is enhanced, had a better opportunity to hear quite distant thunderstorms that never produced rain at the observing station. Another factor is the generally drier western atmosphere that acts to decrease storm rainfall at the surface by increasing the evaporation between the subcloud layer and the surface (Barnes and Newton 1981). However, neither of these factors explain the relatively high incidence of dry thunderstorm days along the Gulf coast where thunder days with no rain occur annually. Most of these days occurred in summer. This area commonly is overrun by low-level, marginally unstable maritime air masses from the Gulf and Atlantic, which, with daytime heating and horizontal convergence from sea breezes, lead to isolated single cell storms that are small and short lived (Barnes and Newton 1981). They further show that most summer thunderstorms in the Gulf area are the airmass type, whereas in the Midwest migratory synoptic-scale systems cause most thunderstorms and only 15% of the summer thunderstorms are the isolated airmass type (Changnon et al. 1977). These small, often isolated storms along the Gulf coast are the primary reason behind the numerous occurrences of thunder heard without any rain at a weather station. Elsewhere in the nation, 3 7 days a year, on the average, have thunderstorms without measurable precipitation Vol. 82, No. 9, September 2001

7 related rainfall. In the central section of the nation where thunderstorm rainfall is a significant part of the annual total precipitation (Fig. 3), a good relationship would be expected. As noted above, the thunderstorm rainfall at Kansas City explained 86% of the variability in the total annual precipitation (Fig. 4a), but in areas of the West where thunderstorm precipitation is a much lessor contributor to the annual total, the effect of thunderstorm rainfall in dry years would appear to be negligible. However, as shown at Helena, where thunderstorm rainfall is only 25% of the annual total, in the six years when the total precipitation was lowest (Fig. 4b), thunderstorm rainfall was also quite low, less than 7 cm. To assess this relationship across the nation, the driest five years in the period, representing the lowest 10%, were identified at each station, and the thunderstorm conditions in those years were compared to the below average values of the total precipitation. Table 3 shows the values determined for Madison. The five annual thunderstorm precipitation FIG. 5. Maximum and minimum annual percentages of thunderstorm rainfall, based on expression as part of the total annual precipitation, and determined for the period. The average annual frequencies of days with no rain on thunderstorm days, expressed as a percent of the average total days with thunderstorms (Fig. 6b), amplifies the mountainous incidence of dry thunderstorm days. More than 30% of all thunderstorm days in that region have no rain. Reno had dry conditions on 42% of its thunderstorm days, and 38% of the storm days at Phoenix and Salt Lake City were dry. Values decrease rapidly away from the Rocky Mountain high, being 10% 15% elsewhere in the nation. Most thunderstorms in these areas occur in multistorm systems such as squall lines and mesoscale convective complexes (Cotton 1999), which reduce the likelihood of hearing thunder without rain occurring. Along the Gulf coast, the dry thunderstorm day values related to frequent airmass storms are 20% of the total incidences. b. Thunderstorm rainfall and dry years The role of thunderstorm rainfall in dry years was assessed. Huff and Changnon (1963) found that the major Illinois droughts during were associated with major decreases in thunderstorm activity and FIG. 6. Measures of the annual number of thunderstorm days without measurable rainfall. Bulletin of the American Meteorological Society 1931

8 TABLE 3. Precipitation and thunderstorm conditions in the five driest years during at Madison, WI. Percent of Percent of Total Percent of average average no. precipitation average annual of thunder Rank Year (cm) precipitation thunder rain days Means values, expressed as a percent of the 45-yr average, matched well with the percentage departures of the annual total amounts, but the numbers of thunderstorm days were not well related with the departures in the annual precipitation, being above average in two of the five years. This analysis of the data for all stations revealed that most dry years occurred in the droughts of the mid-1950s, early 1960s, or late 1980s. The results from the assessment of the 5-yr average departures (below the 45-yr averages) are illustrated by those obtained for 12 stations distributed across the nation (Table 4). Comparison of the two thunderstorm values with the average percent of the total average precipitation at all 51 stations revealed the annual degree of dryness was not related well to the degree of the departures exhibited by the frequency of storm days. However, the annual precipitation percentages were closely related to those of thunderstorm rainfall. Importantly, this good relationship was found at all stations including those in the West and Northeast where thunderstorm precipitation is not a large contributor to the total precipitation (Fig.3). For example, at Portland, Maine, the average thunderstorm precipitation for represents only 13% of the annual average total precipitation. However, as shown in Table 4, the average percentage of the long-term average thunderstorm rainfall in the driest five years was 77%, and this related well to the average of 71% for the total annual precipitation. In Seattle where thunderstorm rainfall represents on average only 5% of the annual total, there also was a close relationship in the departures below average for the dry years, 77% and 79%, respectively (Table 4). Thus, although thunderstorm rainfall is TABLE 4. The averages based on the percent of long-term average for 1) total precipitation, 2) annual thunderstorm precipitation, and 3) annual number of days with thunderstorms, as determined for the five driest years during at selected stations. Percent of annual Percent of annual Percent of average average total average thunder annual number of Station precipitation storm rainfall days with thunder Denver Bismarck Phoenix Seattle Reno Dallas Peoria Memphis Sault Ste. Marie Jacksonville Harrisburg Portland Vol. 82, No. 9, September 2001

9 not a major contributor to the annual total in Seattle or Portland, the degree of decrease in the thunderstorm rainfall in dry years matched the degree of decrease in the total precipitation. At most stations the difference between the means of the two conditions was less than 5%. The correlation coefficient based on the 51 matched values was 0.91 indicating that the departure below average of thunderstorm rainfall in dry years explained 83% of the variation in the total precipitation in the dry years. Hence, most droughts in the nation are related to deficient thunderstorm rainfall. The relationship of dry years and thunderstorm precipitation was further investigated using a different approach for selecting dry years. The five years during with lowest annual thunderstorm precipitation were identified at each station, and their departures from average were computed and compared with departures in the total precipitation for the same five years. This comparative analysis revealed that two different regional relationships existed. In the nation s south-central region where thunderstorm rainfall is 50% or more of the average annual total precipitation (Fig. 3), a close agreement existed for the magnitudes of the two sets of departures. At all 28 stations within this region, the average departure of the five lowest thunderstorm rainfall years was within 10% of the average departures of the total precipitation. For example, at Dallas the average of the five lowest thunderstorm values was 57% of the long-term average, and that based on the five annual precipitation departures of the same years was 62%. Elsewhere in the nation the relation of the two sets of departures was weaker. The average values from the five driest thunder rain years at the weather stations differed by 20% 60% from those derived for the total precipitation. In general, the departures below average for the total precipitation were not as great as those found in the thunderstorm rainfall of the five driest years. For example, at Raleigh the average percentage of the thunderstorm rainfall departures below the 45-yr average for the five driest years was 66% and the average for the total precipitation departures (based on the same five years) was 92%. Nevertheless, at all such stations, the five driest thunderstorm rainfall years were always associated with below average total precipitation. c. Wet years and thunderstorm rainfall The relationship of the total precipitation in extremely wet years and the magnitude of the associated annual thunderstorm rainfall was measured to assess the role of the thunderstorm rainfall. The five years during with the highest amounts were identified at each station. The total precipitation and thunderstorm rainfall for these years were expressed as a percent of the 45-yr averages of each. The magnitude of the two sets of values were compared, and these are illustrated by the values for three stations (Table 5). Inspection reveals that the thunderstorm values were TABLE 5. Comparison of the departures from averages of the annual amount of total precipitation and the annual total thunderstorm rainfall for the wettest five years at three stations, expressed as percent of 45-yr averages. Reno New Orleans Raleigh Total Thunder Total Thunder Total Thunder Rank* precipitation rainfall precipitation rainfall precipitation rainfall Means *Ranks based on the total annual precipitation values for Bulletin of the American Meteorological Society 1933

10 not always in the same rank position as were the total precipitation values. For example, at Reno, the fourth wettest year had 150% of the average precipitation, but the thunderstorm rainfall that year was 229% and the highest of the five storm values. Regardless, the means of the 5-yr values at Reno were close with 159% for total precipitation and 150% for the five thunderstorm values. Good agreement is also shown by the mean departures of New Orleans and Raleigh (Table 5). A comparative analysis of the means for the 51 stations revealed similar values, as shown by the values in Table 5, and the greatest difference found at any station was 10%. In all cases, the five thunderstorm rainfall values were above average. The wet year results reveal, as did the dry year analysis, that the magnitudes of the departures in the annual thunderstorm rainfall closely agree with the magnitudes of the total precipitation. This further illustrates that the extremes in thunderstorm rainfall play a major role in creating the extremes in annual total precipitation. d. Seasonal thunderstorm rainfall Seasonal frequencies of thunderstorms reveal the highest average values occur in summer in most parts of the nation except for the western sections of California and in the extreme western portions of Oregon and Washington. In these areas, large Pacific storms bring most of the region s thunderstorms during the colder half-year. Winter or fall is the peak season of thunderstorm activity at most locations along the Pacific coast, and spring storms create the peak season slightly farther inland. Since summer storm frequency predominates over most of the nation (Changnon 1988), summer thunderstorms should be major contributors to the total summer precipitation at most locations across the nation. The pattern based on the summer (Jun Aug) average rainfall from thunderstorms (Fig. 7a) has a peak of 30 cm or more along the Gulf coast and Florida. The peak of 35 cm or more of rainfall in Florida reflects enhancement of summer convergence over the state s peninsula by the double sea breezes, resulting in frequent convection and numerous thunderstorms (Byers and Braham 1949). Values are also high in the Mississippi River valley, being 20 cm or more as far north as Wisconsin. A secondary high is centered in Missouri and areas to the east where nocturnal thunderstorms maximize in summer due to the influence of the low-level jet in this area (Means 1944). Values diminish rapidly across the high plains, becoming 5 10 cm in the Rocky Mountains and less than 1 cm along the West Coast. The pattern resembles the Easterling and Robinson (1988) summer pattern based on the probability of daily precipitation of 20 mm or more from a thunderstorm. The pattern based on the average summer rainfall from thunderstorms, expressed as a percent of the total average summer precipitation (Fig. 7b), shows an extensive area from Texas to Ohio where more than 80% of the total summer rainfall is produced by thunderstorms. Much of this area exists where the nation s center of mesoscale convective complexes (MCC) exists (Cotton 1999) and where nocturnal thunderstorm activity maximizes (Means 1944). Values are 60% 70% over much of the remaining parts of the nation, and only the West Coast, Northwest, and Northeast have values less than 50%. The average based on the percentages for the stations in the eastern two-thirds of the nation is 72%. Summer storms in the northern Intermountain West area produced between 50% and 70% of the summer total, but in the Southwest the additional storms associated with the FIG. 7. Measures of the average summer rainfall Vol. 82, No. 9, September 2001

11 summer monsoon (Hales 1974) increased the thunderstorm contributions from 70% to 80%. Cotton (1999) reported that thunderstorms in MCCs account for more than 50% of the growing season rainfall in the high plains. Summer thunderstorm rainfall is sufficiently large to serve as an important component of the annual total precipitation. The average summer rainfall values were expressed as a percent of the annual average total precipitation for , and the resulting pattern (Fig. 8) shows that much of the area east of the Rockies has values exceeding 25%, reaching highs of 30% or more in the high plains, western Midwest, and Florida. These findings further reveal that atmospheric conditions that cause a major decrease in summer thunderstorms and their rainfall have notable effects on the total precipitation for the year in the eastern two-thirds of the nation. The average thunderstorm rainfall for the spring season (Mar May) has a maximum of greater than 25 cm in the Oklahoma Arkansas Tennessee area (Fig. 9) with 5 cm or more over most of the eastern half of the nation. Averages are less than 5 cm over the western half of the nation and less than l cm along the West Coast. The spring average values in the Texas Oklahoma Arkansas area were higher than any other seasonal values. In this area, the number of annual thunderstorm days peaks in May but the spring storm frequencies rank a close second behind summer values. The spring rainfall pattern resembles the spring pattern of elevated thunderstorms (Colman 1990), revealing many storms are a result of conditions aloft and not due to surface heating. Figure 10 presents the spring and summer patterns based on the seasonal average thunderstorm rainfall FIG. 9. Average thunderstorm rainfall in spring (cm). amounts expressed as a percent of the annual average thunderstorm rainfall. The spring pattern (Fig. 10a) reveals that values were in excess of 30% in the nation s south-central sections and in portions of the West where spring thunderstorm frequencies are relatively high. However, over most of the United States, the spring average storm rainfall values were between 20% and 25% of the annual thunderstorm totals. The lowest spring values, those less than 20%, occur in the FIG. 8. Percent of the total annual precipitation produced by summer thunderstorm rainfall. FIG. 10. Percent of the average annual thunderstorm rainfall produced by thunderstorm rainfall in spring and summer. Bulletin of the American Meteorological Society 1935

12 northern latitudes and in the Southwest where summer and fall values are high. The summer pattern (Fig. 10b) is almost the reverse of the spring pattern. Low values, less than 40%, exist in the south-central region where spring values were highest, and the highest summer percentages, those greater than 60%, exist in the north-central mountains and high plains. Summer thunderstorm rainfall values decrease to less than 20% of the annual storm totals along the West Coast in areas where spring values were higher. Summer average rainfall values at most stations were the highest of the four seasonal values. As noted, spring values ranked first in the southern plains, Arkansas, and western Tennessee. Winter average rainfall amounts ranked highest along parts of the West Coast. Fall thunderstorm rainfall amounts did not rank highest at any station and were typically the third largest seasonal values. The fall (Sep Nov) thunderstorm rainfall values, expressed as a percent of the annual total thunderstorm rainfall, were used to construct their national pattern (Fig. 11a). Values are low, less than 15%, in the northern mountains and high plains, but exceed 20% in parts of the central and southern sections of the nation. High values, those over 25%, are found in the Southwest, Texas, and Florida peninsula. Fall amounts ranked second highest of the seasonal values in five areas. One included portions of the California coast, and a second was areas downwind of Lakes Superior, Michigan, and Erie where lake effects enhance storm activity in the fall (Changnon 1966). Sault Ste. Marie received 25% of its annual thunderstorm rainfall in the fall, and this value ranks second behind the summer value. The other areas where fall values are ranked second included southeast Texas and Florida where tropical storm activity enhances thunderstorm rainfall production in the fall months, and the fifth area is in the Southwest where the summer monsoon produces added storm activity into September and October. The belt of greater than 20% that extends north south through the Midwest is where Colman (1990) found a peak in elevated thunderstorm occurrences in the fall. The winter (Dec Feb) average thunderstorm precipitation amounts, expressed as a percent of the annual total storm rainfall (Fig. 11b), reveal a high along the West Coast, maximizing at 49% at San Diego. A secondary high exists in the lower Mississippi valley and Colman (1990) showed that these winter storms are largely elevated-type thunderstorms. Winter thunderstorm rainfall in the northern plains where winter thunderstorms are rare events (Court and Griffiths 1981) is less than 1% of the annual total. 4. Temporal findings FIG. 11. Percent of the average annual thunderstorm rainfall produced by thunderstorms rainfall in fall and winter. Linear trends were fit to the values of annual thunderstorm rainfall at the 51 stations, and examples are shown in Fig. 12. The Peoria distribution (Fig. 12a) is typical of others found in the upper Midwest, showing an unchanging trend over time with values ranging from a high of 85 cm in 1965 to a low of 33 cm in The distribution for Salt Lake City (Fig. 12b) resulted in a marked upward linear trend that was statistically significant at the 2% level. Linear values increased from 11 cm in 1950 to 16.7 cm by 1994, a 52% increase. This distribution and trend were typical of those found at stations throughout the western mountains. The distribution for Raleigh (Fig. 12c) results in a downward linear trend. A regional analysis of the 51 linear trends revealed there were five distinct regions (Fig. 13). A large portion of the nation had 45-yr trends that were upward Vol. 82, No. 9, September 2001

13 time. The widespread area of upward trends in thunderstorm rainfall also agreed reasonably well with an analysis of the trends in total precipitation across the nation (Karl et al. 1995). Their analysis of the data for total precipitation showed increases in most areas of the nation including the Midwest. There the thunderstorm rainfall was essentially unchanging with time, but periods of trend analysis of the two studies differ, one beginning in 1910 and the other in 1950, and this may account for the noted regional differences. Recent investigations of national and regional precipitation trends have also found upward trends in heavy rain days for (Karl and Knight 1998) and in 7-day periods of heavy rainfall for (Kunkel et al. 1999). Hence, the temporal distribution of days with thunderstorms when and 5.1-cm amounts occurred was examined for the 51 stations. The national pattern based on the average annual number of thunderstorm days is in close agreement with that for the average number of rain days with > 2.54 and > 6.35 cm (Hydrometeorological Section 1947), suggesting that thunderstorms are the primary cause of FIG. 12. Distributions of annual thunderstorm rainfall amounts for for most heavy rain events. Peoria, Salt Lake City, and Raleigh, with linear trends fit to their distributions. The heavy rain values for the 51 stations were grouped into six major geographical regions based on the values for thunderstorm Those in the Intermountain West region and northern high plains increased between 38% and 55% and were significant at the 1% or 2% level. Large areas with upward, but nonsignificant, trends existed elsewhere across the nation with the increases ranging from 10% to 27%. Stations in the upper Midwest Great Lakes region had nearly flat trends with 45-yr changes of less than 5%. Two stations in the Southeast had downward trends of 10% and 12%. The regional outcome of trends matched well with a trend analysis based on the number of thunderstorm days during (Changnon 2001a). The number of storms had increased throughout the western twothirds of the nation and the South, but had either decreased or were unchanging in the Midwest and Northeast. This close agreement with the pattern based on rainfall trends (Fig. 13) suggests that the observed rainfall increases were a result of more storms over FIG. 13. Regions defined by the linear trends for in annual thunderstorm rainfall. Shown are the highest and lowest percentage changes, based on linear trends for , found at stations within the designated areas. Bulletin of the American Meteorological Society 1937

14 TABLE 6. Regional temporal percentage changes in the frequency of thunderstorm days with heavy rainfall during , as based on values from linear trends fit to regional averages. Percent Percent Regions of rain days > 2.54 cm of rain days > 5.1 cm West Coast Intermountain West High plains Midwest South Northeast rainfall (Fig. 2a) to assess the temporal changes in thunderstorm days with heavy rainfall. Linear trends were fit to the average values for each region, and the changes from 1950 to 1994 were measured and expressed as percent of the 1950 value (Table 6). These values reveal sizable increases in the 2.54-cm days in the West Coast and Intermountain West region, 130% and 116%, respectively, and both were statistically significant. The 45-yr change in 2.54-cm days at Phoenix was from 0.33 days per year in 1950 to 0.8 days by 1994, and values at San Diego went from 0.13 to 0.33 days per year. The Northeast had sizable upward trends for both and 5.1-cm days over time, and the incidence of 2.54-cm days at Harrisburg went from 2.7 days in 1950 to 3.8 days by Trends in both heavy rain classes were also upward in the high plains, Midwest, and South, but the magnitudes of change were slight, 2% 11% for the 45-yr period. For example at Jacksonville, the number of 2.54-cm or heavier rain days went from 10.3 in 1950 to 11.1 in 1994, and at Peoria the change was from 5.7 to 5.9 days. In all regions the increases in the 2.54-cm events were much larger than those for the 5.1-cm events. At many stations the increases in 2.54-cm days were less than those for the number of thunderstorm days during , but in the western third of the nation, Midwest, and Northeast, the magnitude of the heavy rain increases exceeded those found in the number of storm days. In general, the regional outcome for heavy rain days matched well with findings from the earlier studies of temporal fluctuations in heavy rain incidences. However, the lesser increases in the number of thunderstorm heavy rain days in the high plains and Deep South suggest that the sizable upward trends found in thunderstorm rainfall in those regions were more a result of added thunderstorm activity over time and to a lesser extent a result of increased heavy rain events. 5. Summary and conclusions Thunderstorm rainfall was calculated using daily totals on days with thunderstorms that had been modified (reduced) by using ratios determined from a comparative analysis of the hourly amounts when thunderstorms occurred with the daily totals on thunderstorm days. The annual ratios at stations across the nation ranged from 86% to 95%. Hourly rain totals with thunderstorms could not be used directly for estimating thunderstorm rainfall because a substantial number of days with thunderstorms had no hourly records of thunderstorm occurrences. The average annual thunderstorm rainfall pattern matches the average number of thunderstorms pattern, with more than 70 cm of thunderstorm rainfall in the Southeast. Values diminish to 20 cm along the Canadian border, and become 15 cm along the Front Range of the Rockies. Average thunderstorm rainfall totals are least, less than 5 cm, along the West Coast. The daily average rainfall amounts on a thunderstorm day peak in the lower Mississippi River valley, an area where cloud-to-ground lightning flashes are at a national maximum. The nation s lowest average daily amounts occur in the Intermountain West region with only 2 4 mm per storm day. This high frequency of summer thunderstorms but little rainfall production from storms explains why this region is susceptible to lightning-induced fires. This mountain area averages more than 11 days a year with thunderstorms and no rain, as compared to 3 7 such dry thunder days at most points elsewhere in the nation. Isolated airmass thunderstorms common in the Gulf coast area also led to numerous days with thunder but no rain at the weather stations. The annual thunderstorm rainfall is greater then 60% of the annual average total precipitation amounts in the lower 1938 Vol. 82, No. 9, September 2001

15 and central Mississippi River valley, retreating to 20% or less of the total amounts in the West and Northeast. Thunderstorm rainfall values across the eastern twothirds of the nation average 47% of the region s total precipitation. The results reveal that thunderstorm rainfall is sizable across most of the eastern two-thirds of the nation, representing 50% 70% of the annual total amounts. Annual thunderstorm rainfall amounts relate well to the annual total precipitation with correlation coefficients of 0.75 or higher in the central third of the nation and ranging from 0.5 to 0.6 elsewhere. The magnitudes of droughts across the entire nation, based on the percent of average precipitation for the five driest years during , were closely related to the percentage decreases found in thunderstorm rainfall. A comparable analysis of the five wettest years during revealed a similar relationship at all 51 stations. The magnitudes of the mean values of the five above average values in total precipitation were within 10% of the means of the thunderstorm rainfall in these wettest years. Thus, the magnitudes of extremely wet and dry years anywhere in the nation are closely related to the magnitude of thunderstorm rainfall, reflecting the major influence of thunderstorm rainfall on extremes in total precipitation. Summer thunderstorm rainfall is the highest seasonal amount across most of the United States, producing on average 50% 80% of the total summer rainfall and 20% 40% of the annual total precipitation over most of the United States. Spring thunderstorm rainfall averages are the highest of any season in the southern plains (TX, OK, AR), and winter averages are highest along portions of the West Coast. Summer storm rainfall is 60% 75% of the annual total thunderstorm rainfall in the northern mountains and high plains, and spring amounts are 30% 45% of annual storm totals in the Texas Oklahoma area. Winter storm rainfall at various locales on the West Coast ranges from 31% 49% of the annual storm totals. Fall amounts rank as the second highest seasonal totals in portions of California, the Southwest, in southeast Texas and Florida, and downwind of the Great Lakes in portions of Michigan, Ohio, and New York. The temporal distribution of thunderstorm rainfall for the period was upward over large portions of the nation except for the upper Midwest and a small part of the Southeast. The rainfall increases in the Intermountain West area and northern plains ranged from 38% to 55% over the 45-yr period and were statistically significant. The various regions of increases and decreases in thunderstorm rainfall relate well to trends in thunderstorm frequencies. Temporal incidences of heavy storm rainfall events exhibited increases across the entire nation and heavy rain days increased considerably in the West Coast, the Intermountain West region, and the Northeast. Hence, the widespread shifts to more thunderstorm rainfall over time were the result of added storm incidences plus more heavy rain events over time. Major features in the various thunderstorm rainfall patterns were found to be related to the major factors that influence storm activity including MCCs, unstable airmass convection, elevated thunderstorms, lake- and sea-breeze effects, the southwest summer monsoon, and Pacific storms of the cold season. This determination of the average and extreme precipitation amounts produced by thunderstorms will allow assessments of the value of this precipitation in agriculture, water resources, and ecosystems. Thunderstorms contribute more than half of the total precipitation over two-thirds of the nation and thus are critical elements in the hydrologic cycle. This is reflected by the close association of the departures from average of wet year and dry year precipitation with those found for thunderstorm rainfall. Thunderstorms produce 70% of the average summer rainfall in most major crop growth areas including the high plains, Midwest, South, and East. The dry thunderstorms of the summer with many storms without measurable rainfall help explain the forest fire problem of the West. Obviously, thunderstorm precipitation is a critically important part of the water cycle of North America. Acknowledgments. This research was funded by a grant from the National Oceanic and Atmospheric Administration (NOAA) and the Department of Energy (DOE), as part of the Climate Change Detection and Attribution Project, NA96GP0455. The views expressed herein are those of the author and do not necessarily reflect the views of NOAA or DOE or any of their subagencies. The assistance of Bob Scott and Jan Smith is gratefully acknowledged. References Barnes, S. L., and C. W. Newton 1981: Thunderstorms in the synoptic setting. Thunderstorm Morphology and Dynamics, Vol. 2, Thunderstorms: A Social, Scientific, and Technological Documentary, E. Kessler, Ed., Environmental Research Laboratory, NOAA, Washington, DC, Byers, H. R., and R. R. Braham, 1949: The Thunderstorm. U.S. Government Printing Office, Washington, DC, 287 pp. Bulletin of the American Meteorological Society 1939

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